The present invention is directed to a method of and apparatus for improving the recording process used with magneto-optical disks. More particularly, the present invention is directed to a method of and apparatus for the direct overwriting on single-layer magneto-optical disks by the use of a circularly polarized microwave magnetic field in combination with a laser beam.
The direct overwriting of recorded data contained on a single-layer magneto-optical disk is highly desirable since it allows recording of data directly onto such a disk without having to first erase the recorded data. While direct overwriting systems are known, they have certain drawbacks. For instance, double layer (or trilayer) exchange-coupled magnetic media are difficult to fabricate, and tradeoffs that need to be made in order to bring these exchange-coupled devices to the proper operational range, are costly in terms of signal-to-noise ratio during readout. Also, direct-overwrite devices based on the technique of magnetic field modulation, require a low-flying magnetic head on the side of the magneto-optical disk opposite to the optical head (i.e., objective lens, laser, beam-splitters, etc.). These low-flying magnetic heads eliminate one of the main advantages of optical recording, known as disk removability; they also restrict the disks to be single-sided.
In magneto-optical recording, the recording system operates by writing information signals on a magneto-optical recording medium, such as a magneto-optical disk, by the application of heat generated by a laser light. Magnetization direction is generally determined by a magnetic field which is applied to the heated portion of the disk upon cooling. One of the advantages of such magneto-optical recording systems over other types of optical disks is that it is possible to rewrite the information signals. In other words, in magneto-optical recording, new data can be written onto a disk in an area of the recording medium in which a recording pattern or data was previously written.
In the prior art, the rewriting of data on a magneto-optical disk has typically been accomplished by inverting the bias magnetic field. Typically, a permanent magnet having more than a predetermined magnetic field intensity is utilized to provide excitation of the magnetic field. In using such a method, the recording and erasing processes are accomplished as two separate steps. For example, the magnet must be mechanically inverted in its position after the first erasure for recording on the appropriate area of the magnetic recording medium from which the previous recording has just been erased. An alternative method, which is used more often, is an electromagnet which can be switched by a current source. No mechanical motion of the magnet is involved. But, because this type of electromagnet is large, it cannot be switched rapidly. Thus it is switched once to erase an entire track, and is then switched a second time to record on the erased track.
It is also known in the prior art to use separate erasing and recording devices in combination in order to effect the overwrite function. In such known methods, erasing and recording are accomplished on different portions of the magneto-optical recording medium. However, it is very difficult to accomplish real-time recording using the two methods described above.
There is also known in the prior art a device for producing a high frequency modulation magnetic field which may be used in magneto-optical recording in order to effect real-time overwriting of previously recorded data. For example, U.S. Pat. No. 4,796,241, to Hayakawa et al., issued Jan. 3, 1989, describes a device for producing a high frequency modulating magnetic field which is especially useful for achieving real-time overwriting in magneto-optical recording. The device produces a magnetic field modulated according to the signal to be recorded on a magneto-optical recording medium in which the signal is recorded in the form of a magnetization direction on the magneto-optical recording medium under irradiation of a laser beam in order to raise the temperature of the medium.
The device disclosed by Hayakawa et al. includes a main magnetic pole formed of a low-loss soft magnetic material having one end facing the magneto-optical recording medium and a sheath conductor wound around the main magnetic pole which consists of a parallel connection of a plurality of insulated elementary wires.
Another example of a device which provides for the direct overwriting of data previously recorded on a magneto-optical recording medium is shown in U.S. Pat. No. 4,907,211 issued Mar. 6, 1990, to Horimai et al. In the device of Horimai et al., the magneto-optical recording medium is irradiated with a laser light corresponding to an information signal only when a magnetic field sufficient to invert the magnetizing direction of the magneto-optical recording medium is applied. The device of Horimai et al. prevents a high-noise region from being formed such that a high density recording can be accomplished.
An example of a non-direct overwriting system is shown in U.S. Pat. No. 4,855,975 to Akasaka et al., issued Aug. 8, 1989. That patent discloses a magneto-optical recording method for recording data on a recording layer of a multi-layered magnetic recording medium having a first layer having a perpendicular magnetic anisotropy which acts as a recording layer and a second layer having a perpendicular magnetic anisotropy which acts as a reference layer. However, the device of Akasaka et al. utilizes two adjacent laser beams, in addition to the magnetic field producing device. Thus, it is far more complex than either of the two devices described above, or that of the present invention.
The present invention, on the other hand, provides for the direct overwriting on single-layer magneto-optical disks by the use of a circularly polarized microwave field in combination with a laser beam in order to reverse the magnetization of a small region heated by the laser beam. Such a system has several advantages over the known techniques discussed above. For example, the microwave magnetic field utilized to achieve the magnetization reversal of the magneto-optical disk does not have to be sharply focused.
Another advantage of the present invention is that the magnetization of the heated region can be turned to a desired direction by applying the microwave magnetic field of the proper sense of polarization and amplitude, regardless of the initial direction of the magnetization of the heated region, i.e., regardless of the initial direction of the already recorded data which is being overwritten. This feature allows the overwriting of data to be accomplished without having to first read the recorded data. The magneto-optical overwriting system of the present invention allows the direct overwriting of previously recorded data on a single layer magneto-optical disk using a circularly polarized microwave magnetic field that applies to perpendicular magneto-optical media.
The present invention relies on its ability to switch the direction of magnetization in a perpendicular magnetic thin film (such as that used with magneto-optical disks) using an in-plane oscillating magnetic field. The frequency of oscillations are generally in the neighborhood of the ferromagnetic resonance frequency of the thin film material.
While it is known to utilize linearly polarized microwave fields with, for example, bubble domain memories, such as that disclosed by D. J. Seagle, J. O. Artman and S. H. Charap, Bubble Generation by Microwaves: Analytical, J. Appl. Phys. 55 (6), Mar. 15, 1984, pp. 2578-2580 and J. O. Artman, S. H. Charap and D. J. Seagle, Microwave Generation of Bubble Domains and Magnetic Thin Films, IEEE Transactions on Magnetics, Vol. Mag.-19, No. 5, September 1983, pp. 1814-1816. Neither of those two articles, however, disclose nor suggest, the use of circularly polarized microwaves for use with magneto-optical disks.
It has also been proposed to use circularly polarized optical radiation in order to change the optical transmission characteristics of polarized light through a ferromagnetic thin film such as that used for magnetic bubble devices. Such a method and apparatus is disclosed in
U.S. Pat. No. 4,424,580 to Becker et al., issued Jan. 3, 1984. Becker et al. discloses such methods and apparatus for reversibly altering the magnetic characteristics or domains of ferromagnetic material by photomagnetic means, particularly by electron spin reversal.
In the reported experiments by Messrs. Seagle, Charap and Artman, as well as others, the wave radiation is delivered to a small area of the magnetic film in order to generate reverse-magnetized domains. However, all of such methods and systems cannot tell the initial state of the magnetization. That is, since the microwave radiation utilized is linearly polarized, the magnetization state of the recorded data could be either "UP" or "DOWN". However, the present invention, by utilizing circularly polarized microwave fields, can utilize the right sense of polarization to switch from the "UP" to the "DOWN" state and the left sense of polarization to switch from the "DOWN" to the "UP" magnetization state in a unique and definite manner.
Thus, the present invention, by replacing the electromagnet of a conventional thermomagnetic recording system with a microwave source that delivers a fixed frequency AC in-plane magnetic field to a small region of the disk is able to overwrite the initial state of magnetization of the magneto-optical disk.
An advantage of the present invention is that the microwave field does not have to be sharply focused, since the recorded domains are not defined by the area exposed to the microwave field itself. The recorded domain is determined by the size of the focused laser beam, as is conventional in thermomagnetic recording. Further, there is no need to fine-tune the frequency of the microwave field, since the temperature variations induced at each location on the disk as the laser beam passes over that location will vary the effective internal field on the dipoles and thereby change the resonance frequency.